3GPP LTE represents the project within the third generation partnership project, with an aim to improve the UMTS standard. The 3GPP LTE radio interface offers high peak data rates, low delays, and an increase in spectral efficiencies. The LTE system supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD). This enables operators to exploit both the paired and unpaired spectrum, since LTE has flexibility in bandwidth as it supports 6 bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz.
The LTE physical layer is designed to achieve higher data rates. This is facilitated by turbo coding/decoding, and by higher order modulations (up to 64-QAM). The modulation and coding is adaptive, and depends on channel conditions. Orthogonal frequency division multiple access (OFDMA) is used for the downlink, while single carrier frequency division multiple access (SC-FDMA) is used for the uplink. The main advantage of such schemes is that the channel response is flat over a sub-carrier, even though the multi-path environment could be frequency selective over the entire bandwidth. As a result, the complexity involved in equalization is reduced, as simple single tap frequency domain equalizers can be used at the receiver. OFDMA allows LTE to achieve its goals of higher data rates, reduced latency, and improved capacity/coverage, with reduced costs to the operator. The LTE physical layer supports Hybrid Automatic Repeat Request (HARQ), power weighting of physical resources, uplink power control, and multiple input multiple output (MIMO). By using multiple parallel data stream transmission to a single terminal, the data rate can be increased significantly.
MIMO is an advanced antenna technique to improve spectral efficiency, thereby boosting the overall system capacity. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configurations in terms of the number of transmit antennas (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (2×4), (4×4), (8×4). The multiple input single output (MISO) and single input multiple output (SIMO) configurations represented by (2×1) and (1×2), respectively, are special cases of MIMO.
It is well known that MIMO systems can significantly increase the data carrying capacity of wireless systems. MIMO can be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. For these reasons, MIMO is an integral part of the 3rd and 4th generation wireless systems. In addition, massive MIMO systems are currently under investigation for 5G systems.
FIG. 1 is a schematic diagram of multi-antenna transmission in LTE. More particularly, FIG. 1 illustrates data modulation 5A and 5B, antenna mapping 10, antenna ports 15, OFDM modulator 20, and antennas 25. Antenna mapping 10 can, in general, be described as a mapping from the output of data modulation 5A and 5B to different antenna ports 15. In the example illustrated in FIG. 1, there may be up to eight antenna ports 15. The input to antenna mapping 10 consists of modulation symbols (e.g., QPSK, 16QAM, 64QAM, 256QAM etc.) corresponding to the one or two transport blocks. More specifically, there is one transport block per Transport Time Interval (TTI), except for spatial multiplexing, in which case there may be two transport blocks per TTI. The output of the antenna mapping 10 is a set of symbols for each antenna port 15. The symbols of each antenna port 15 are subsequently applied to the OFDM modulator 20. In other words, the symbols of each antenna port 15 are mapped to the basic OFDM time-frequency grid corresponding to that antenna port 15. The output of OFDM modulators 20 may then be transmitted by antennas 25. For example, data may be transmitted by antennas 25 to a user equipment (UE).
FIG. 2 illustrates an example signal flow diagram for downlink data transfer in LTE. At step 205, UE 110 receives pilot or reference signals transmitted by network node 115, such as an eNodeB. From the pilot or reference signals, UE 110 computes channel estimates, and then computes the parameters needed for channel state information (CSI) reporting. The CSI report may include, for example, a channel quality indicator (CQI), a precoding matrix indicator (PMI), and rank information (RI) (see TS 36.213 V8.8.0, section 7.2).
At step 210, UE 110 sends the CSI report to network node 115 via a feedback channel, such as, for example, the physical uplink control channel (PUCCH) or the physical uplink shared channel (PUSCH). The PUCCH may be used for periodic CSI reporting, while the PUSCH may be used for aperiodic reporting. A scheduler associated with network node 115 uses this information in choosing the parameters for scheduling of UE 110. At step 215, network node 115 sends the scheduling parameters to UE 110 in the downlink control channel called physical downlink control channel (PDCCH). At step 220, actual data transfer takes place from network node 115 to UE 110. Data transfer between network node 115 and UE 110 may continue for any suitable period of time. In certain circumstances, however, it may become necessary for UE 110 to be handed over from network node 115 to another network node (i.e., a target network node). The handover (HO) procedure is described in more detail below.
FIG. 3 illustrates an example signal flow diagram of LTE handover. More particularly, FIG. 3 illustrates the Inter-eNode B intra-frequency LTE HO. At steps 302 and 304, downlink and uplink user data is transmitted between UE 110 and source network node 115A. At step 306, UE 110 transmits an RRC MEASUREMENT REPORT A3 (intra-LTE) or A2 (inter-LTE) message. As disclosed in TS 36.331 V8.21.0, section 5.5, UE 110 may send the RRC MEASUREMENT REPORT when UE 110 is on the cell border.
At step 308, source node 115A sends a handover request (over an internode interface, such as X2, if setup), which target node 115B acknowledges at step 310. At step 312, source node 115A sends an RRC Connection Reconfiguration message in response to target node 115B's acknowledgement. The RRC Connection Reconfiguration message is referred to as the signaling radio bearer message (SRB). The SRB instructs UE 110 to the new cell by means of the PCI, carrier frequency, antenna ports, etc.
At step 314, source node 115A sends an eNB_Status_Transfer message to target node 115B. At step 316, UE 110 sends a Random Access Preamble message to target node 115B, and at step 318 target node 115B sends a Random Access Response message to UE 110. At step 320, UE 110 sends a Random Access Msg3 message to target node 115B, and at step 322 target node 115B sends a Contention Resolution (UL Grant) message to UE 110.
At step 324, UE 110 sends an RRC Connection Reconfiguration Complete message to target node 115B when it has added the new radio link (e.g., through contention). Steps 326 and 328 illustrate the exchange of Path_Switch_Request and Path_Switch_Request_Acknowledgement messages between target node 115B and core network node 130. At steps 330 and 332, downlink and uplink user data is exchanged between UE 110 and target node 115B. At step 334, target node 115B sends a RRC Connection Reconfiguration message to UE 110, and at step 336, UE 110 sends an RRC Connection Reconfiguration Complete message to target node 115B. At step 338, target node 115B sends a UE Context Release Command to source node 115A.
When network nodes 115A and 115B are deployed with multiple antennas, and UE 110 is configured to receive the SRB transmission from multiple antennas, the HO performance becomes more critical. This is because there are cases when UE 110 reports higher transmission rank at the cell edges. The interference pattern, however, can change dynamically at the cell edge. For example, at the time of CSI reporting there might be less interference in the neighbor cell 115B, while at the time of data transfer (both data and signaling) there might be high interference. In such a case, packets may get an error, and HARQ might be useful. Retransmitting SRBs, however, is costly in terms of payload, and signaling messages are delay sensitive, which eventually impacts the HO performance.
Typically, HO performance and call retainability are key performance indications (KPIs) when setting up a new mobile network. Dropped calls and long HO interruption would be very annoying for customers. This can badly impact a customer's willingness to continue subscriptions. In both homogeneous and heterogeneous networks (including macro, micro, and pico cells), HOs typically occur on cell edge. Usually, the HO fails due to transmission failure of key HO signaling messages, such as, for example, the RRC Connection Reconfiguration message. Thus, there is a need for a more reliable method of transmitting key signaling messages.